X-ray observations of the 50-min dipping source XB 1916-053.

Abstract

Three EXOSAT observations of the low-mass X-ray binary XB 1916-053 have revealed extreme variability in the appearance of the dips, ranging from complete absence to two broad, deep dips per 50-min cycle. These dips are believed to result from the obscuration of the central X-ray source by azimuthal structure in the accretion disc. The X-ray spectrum during quiescent (i.e. non-dipping, non-bursting) intervals is well described by a simple power-law model with photon index -1.8. Intensity-selected spectra are well fitted by either a simple power law which becomes flatter with decreasing flux, or a composite model with contributions from a heavily-absorbed component and a non-absorbed, probably Thomson scattered component. From an analysis of four bursts observed from XB 1916-053, and the assumption that the peak luminosity in the bursts can be equated with the Eddington luminosity, the authors derive a distance of 8.4 kpc to the source assuming cosmic abundances, or 10.8 kpc if the accreted material is extremely hydrogen-deficient, as required by published evolutionary models.